Methods of two-way communication using the WebSocket standard as a means of exchanging data between a plurality of devices are known from the literature (see, for example, PTL 1). Each time data is stored in the storage unit of the server in the configuration disclosed in PTL 1, data is sent from the server to a client device. This configuration enables transmitting data while omitting the process of the client device requesting data from the server.
POS systems used for processing transactions related to product sales that are configured so that price, quantity, and other information is output by a device other than the POS terminal device used by the operator are also known from the literature (see, for example, PTL 1). A POS customer display that is connected to a host device and displays information according to commands input from the host device is described in PTL 2. Output devices such as customer displays used in POS systems must be able to output information in real-time without creating a significant delay when the transaction process is run by the POS system. As a result, the customer display is conventionally connected directly to the host device as described in PTL 1, and commands are sent as necessary from the host device to control the display.